1. Field of the Invention
The present invention relates to a method and a system for a radio wave propagation characteristics estimation and a ray spatial resolution control, and in particular to a radio wave propagation characteristics estimation method, and a ray spatial resolution control method in predicting the radio wave propagation characteristics by a technique of geometrical optics.
2. Description of the Prior Art
A radio wave propagation simulator is employed to assist the arrangement of a base station or a host system in a radio communication system. The received power or delay spreading at any receiving point can be estimated by using the radio wave propagation simulator. On the basis of this estimation, an installation site of an effective transmitting station is determined, so that the overall efficiency can be enhanced by reducing the number of base stations to be arranged.
The radio wave propagation simulation is largely classified into a statistical technique and a deterministic technique. The statistical technique employs an expression for estimating the propagation loss with the arguments of distance and frequency, in which the parameters in the estimation expression are determined on the basis of a large amount of data resulted from actual measurements of the propagation loss in accordance with the statistical technique including the multivariate analysis. On the other hand, the deterministic technique is one in which the radio wave radiated from an antenna is regarded as an a gathering of a number of radio wave rays and each ray is reflected and transmitted repeatedly on the geometrical optics, and propagated. The propagation loss and the amount of delay at an observation point can be obtained by synthesizing the electric power and the propagation time of the rays incoming to the observation point.
The deterministic technique is further largely classified into an imaging technique and a ray launching technique, depending on a way of keeping track of the propagation path. The imaging technique determines a reflection and transmission path of the ray connecting between the transmitting and receiving points by obtaining an imaging point against the reflection surface. Since the reflection and transmission path is uniquely determined if the transmitting and receiving points and the reflecting and transmitting barriers are decided, the imaging technique can search a strict propagation route of the ray.
On the other hand, the ray launching technique is one in which the rays from the antenna are radiated at discrete angular intervals, irrespective of the position of the receiving point, and the ray passing near the receiving point through the repeated reflection and transmission is regarded as the ray incoming to the receiving point. The ray launching technique solves approximately, but not strictly like the imaging technique, the propagation route of the ray connecting between the transmitting and receiving points, and has a feature of shortening the time needed to search for the propagation route.
With the launching technique, first of all, a closed area is obtained around the transmitting antenna, and divided into the partial spaces. Then, one ray is allotted to each partial space, whereby the partial space and allotted ray are regarded as identical. Thereafter, the propagation routes for a finite number of rays radiated from the transmitting antenna are tracked, and on the basis of its result, a situation of radio wave propagation is estimated in an entire space around the antenna.
One example of dividing the closed area around the antenna into partial spaces was described in a literature written by Scott Y. Seidel, et al: “Site-Specific Propagation Prediction for Wireless In-Building Personal Communication System Design”, IEEE TRANSACTIONS VEHICULAR TECHNOLOGY, VOL. 43, NO. 4, NOVEMBER 1994, pp. 878-891.
In this literature, first of all, a three dimensional closed area of a regular icosahedron is provided around a transmitting antenna 301, as shown in FIG. 12. Next, a face making up the regular icosahedron, namely, a plane of an equilateral triangle composed of the vertexes 406, 407 and 408 is extracted, and each side is divided into equal two lengths, using the points 409, 410 and 411, as shown in FIG. 13. By drawing the line segments parallel to the sides of the equilateral triangle having the vertexes 406, 407 and 408 and passing at the division points, a similar triangle for the original triangle is newly created internally. The above processing is performed for all the faces constituting the regular icosahedron of FIG. 12. Then, the vertexes of each equilateral triangle are moved in the direction of connecting the center of gravity in the regular icosahedron and the vertexes of each equilateral triangle newly created, so that there is an equal distance from the center of gravity, as shown in FIG. 14, for example.
FIG. 14 is a view showing an instance where one side of the equilateral triangle making up each face of the regular icosahedron of FIG. 12 is bisected. Rays radiated from a transmitting antenna 501 located at the center of gravity in the original regular icosahedron are radiated in each directions of connecting the transmitting antenna 501 and the vertexes of the polyhedrons of FIG. 14. In FIG. 14, as one example, a ray 504 passing at a vertex 502 is shown. At this time, a partial space 503 defined from the polygon of FIG. 14 and the transmitting antenna 501 is regarded as identical to the ray 504.
FIG. 15 is a view showing a partial space 605 regarded as identical to a ray 601. At a point 602, the area of a section 603 perpendicular to the ray 601 is defined hereinafter as a spatial resolution at the point 602. The spatial resolution at the point 602 is increased when the length between a transmitting antenna 604 and the point 602, namely, the propagation distance of the ray is greater. In this manner, a wide space is regarded as identical to one ray, whereby the radio wave propagation estimation precision by the ray launching thecnique is decreased with larger propagation distance.
Thus, in order to maintain the spatial resolution at a certain value or below at any time, irrespective of the propagation distance, a method of dividing the ray propagating per a predetermined distance was proposed. One example of this method was described in a literature written by Steven Fortune: “EFFICIENT ALGORITHMS FOR PREDICTION OF INDOOR RADIO PROPAGATION”, in Proceedings of the 48th IEEE Vehicular Technology Conference, May, 1998, pp. 572-576. FIG. 16 is a view for explaining the method as described in the above literature.
A triangular cone 712 of FIG. 16 is devided into a triangular cones 708 to 711, if the propagation distance of an allotted ray 702 from a transmitting antenna 701 reaches a predetermined value. The rays 704 to 707 are allotted to the divided triangular cones, whereby the same processing is repeated subsequently. As a result, the spatial resolution can be kept at a certain value or below at any time, irrespective of the propagation distance.
FIG. 17 is a view for explaining the operation of the ray launching technique in the case where an observation area 017, a transmitting point 015, a receiving point 016, and two objects 001 and 002 within the observation area are provided. In FIG. 17, for the simplicity, the operation is explained only in the two dimensional plane, but it is common that the operation is performed in the three dimensional space.
A ray radiated in a direction along a propagation route 003 at a discrete interval is incident upon the object 001 at a point 018, so that a reflected ray 005 and a transmitting ray 004 are produced. The reflected ray 005 is further incident upon an object 002 at a point 019, so that a reflected ray 006 and a transmitting ray 007 are produced. Since the produced ray 006 passes by the receiving point 016, the ray 006 is regarded as incoming to the receiving point, a path consisting of the paths 003, 005 and 006 is regarded as one of the propagation routes connecting between the transmitting point 015 and the receiving point 016.
Specifically, the receiving power and the incoming delay time acquired from the propagation routes 003, 005 and 006 are recorded in FIG. 18. In FIG. 18, the transverse axis 103 represents the delay time required for the ray to proceed from the transmitting point 015 via the routes 003, 005 and 006 to the receiving point 016, and the longitudinal axis 102 represents the power strength of the ray having passed through the above route. For the transmitting rays 004 and 007, the transmission and reflection are repeatedly searched in the same manner as in the propagation routes 003, 005 and 006. The ray that passes by the receiving point 016 is treated as the incoming ray, as in the propagation route 006, and the above processing is continued till the search end condition is met.
The search end condition occurs when the received field strength at the reflection and transmission point falls below a predetermined value, or the total number of reflection and transmission reaches a predetermined number of times. After the ray radiated from the transmitting point 015 in the direction toward the propagation route 003 is searched for the reflection and transmission routes, the rays radiated in other radiation directions such as the rays 008 to 014 are also searched for the reflection and transmission routes. The rays are radiated in all the directions as defined in advance, and searched for the propagation routes, whereby a delay profile at the receiving point 016 can be obtained as shown in FIG. 19. In FIG. 19, the transverse axis 203 represents the time taken for the ray to go from the transmitting point 015 to the receiving point 016, and the longitudinal axis 202 represents the power strength of the ray having passed through the above route.
The received power at the receiving point 016 is given by a total of power strength for all the paths as indicated in FIG. 19, and the delay spreading indicating the distortion is given by the standard deviation of the delay time with a power ratio of the power strength to the received power in each delay time as the occurrence probability of the delay time.
Since the ray is regarded as identical to the partial space nearby the ray in the launching technique as described above, when the ray is reflected from a barrier, it is considered that the same reflection has occurred in the partial space in the vicinity of the ray. Therefore, from a viewpoint of the estimation precision, it is desirable that the spatial resolution at the reflection point is set not to be too larger than the area of the barrier. However, to improve the estimation precision, if the upper limit value of the entire spatial resolution is set at the minimum value of spatial resolution required when reflecting the ray from the barrier, there is a problem that the calculation time taken for the radio wave propagation estimation is increased. The reason is that if the spatial resolution is reduced, a greater total number of rays are radiated and the total time for searching the route is increased.
On one hand, if the upper limit value of spatial resolution is set to be relatively large to suppress the calculation time required for estimation, there is a problem that the overall estimation precision may be degraded. The reason is that when the ray is reflected from the barrier with a small spatial resolution, it is considered that the reflection also occurs even in the ray near area where the reflection does not actually occur.